This invention relates to a photomask container, more particularly to a container in which photomasks can be transported and stored without damage to their quality.
Photomasks require a very precision in definition and must be stored with utmost care. They are usually stored or transported in containers that are airtight and which feature various geometric designs that prevent photomasks from contacting each other.
Because of ease in molding and fabricating operations, styrenic or acrylic resins have conventionally been used to manufacture photomask containers. Styrenic resins are used with particular preference since they are highly transparent and can easily be formed by injection molding. On the other hand, the containers made of styrenic or acrylic resins have no electric conductivity and, during the storage or transport of photomasks, static charges will build up to increase the chance of a discharge of occurring between the container and photomasks, thereby potentially destroying the photomask pattern. As a further problem, the static charges on the container will attract suspended dust particles in air and, if the container is brought into the Si wafer preparation line, process contamination will occur.
In order to solve these problems, it has been proposed that acrylic resins such as polymethyl methacrylate be rendered conductive by dispersing a conductive acrylic rubber to produce a three-dimensional network in the resin. However, this is not a complete solution to the problems at issue. If acrylic resins are used, monomers such as methyl acrylate and methyl methacrylate will remain in the wall of the molded container and such residual monomers will be liberated from the container during storage or transport of photomasks, with the emitted odor being a potential hazard to the health of the operating personnel. Furthermore, since sodium metasilicate is used to have the conductive acrylic rubber enter into a crosslinking reaction, sodium may be released from the wall of the molded container to adhere to photomasks that are being stored or transported in the container. As a consequence, the transparency of the substrate glass will decrease to cause adverse effects not only on the efficiency of resist development in the Si wafer preparation line but also on the contact exposure of silicon wafers since the releassed sodium may be transferred to the wafers to degrade the quality of a completed IC chip product.